US Patent 7324419 Focus control via AC input signal

A method of mapping a surface of a printable medium comprises illuminating the surface with electromagnetic radiation and focusing the electromagnetic radiation with a focusing means. The method also includes modulating the position of the focusing means using an actuator, wherein the actuator has a resonant frequency. An input signal is provided to the actuator, wherein the input signal comprises an AC waveform having a frequency. The frequency does not correspond to the resonant frequency of the actuator. The waveform does not have higher harmonics corresponding to the resonant frequency of the actuator.